Visualization of invasion–percolation drainage process in porous media using density-matched immiscible fluids and refractive-index-matched solid structures

Liquid water transport in hydrophobic gas-diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs) is indirectly investigated through a similarity model experiment, that visualizes the drainage process in a porous layer. The dimensionless parameters for the visualization experiment are controlled to be close to those for liquid water transport in hydrophobic GDLs by slowly injecting a density-matched immiscible fluid into a porous layer saturated with liquid water. The visual inspection of the drainage process is conducted by constructing the porous layer with saturated hydrogel spheres possessing a refractive index that is almost the same as that of liquid water. The visualization results indicate that the drainage process considered in this study is a strongly capillary-driven process governed by invasion–percolation, this suggests that invasion–percolation with capillary dendrite-like penetration (fingering) can also be an important mechanism of liquid water transport in hydrophobic GDLs. The study also examines the morphological similarities between the non-wetting fluid distribution observed for the present drainage experiment and that predicted by a steady pore-network model for liquid water transport in hydrophobic GDLs.